1. Field of the Invention
This invention relates to an image reading apparatus, an image reading method and a program, and particularly to an image reading apparatus which photoelectrically converts an inputted original image, and converts an analog electrical signal obtained by the photoelectric conversion into a digital signal, an image reading method applied to the image reading apparatus, and a program for causing a computer to execute the image reading method.
2. Related Background Art
Heretofore, in a color image reading apparatus for reading an original image as a multi-color image, an optical signal representative of the image has usually been converted into an electrical signal by a CCD sensor. This CCD sensor is generally comprised of three line sensors, films which provide red (R), green (G) and blue (B) color filters are applied to the surfaces of the respective line sensors, and electrical signals corresponding to the respective colors are outputted by these line sensors.
The color reproducibility of the image read by the color image reading apparatus is determined by the spectral characteristic of a light source and a spectral transmission characteristics of the color filters. The kinds of the color filters include ones using complementary colors, i.e., cyan (C), magenta (M) and yellow (Y), corresponding to an output apparatus such as a printing machine, besides R, G and B, and one corresponding to an XYZ color system using measured color values X, Y and Z which do not depend on a device, and all of these are provided with a view to improve the color reproducibility of the color image reading apparatus.
Generally, a color filter used in the XYZ color system adopts a construction in which many kinds of color filters are combined together, from the feature of the spectral characteristic thereof, and therefore an image processing system becomes complicated.
As technique which can more improve color reproducibility than the RGB three-color system, there are, for example, a method of effecting image reading by four colors, i.e., R, G, B and Ye (yellow), as shown in Japanese Patent Application Laid-Open No. 2003-87583, and a method of effecting image reading by multiple colors through the colors are not specified, as shown in U.S. Pat. No. 6,459,512. The color reproducibility basically has the possibility of being more improved as the number of basic colors expressing an image is increased, and therefore these methods can be said to be effective for improving the color reproducibility. However, a great number of channels lead to the corresponding complication of a subsequent image processing system.
The conversion of an analog signal into a digital signal is effected in order to thereafter carry out shading correction, matrix conversion, gamma correction and various kinds image processing to analog image data read by four channels (four basic colors) on a digital circuit IC (integrated circuit) in the color image reading apparatus or a PC (personal computer).
The final image data, although depending on an output device which becomes a target, is usually converted into RGB three-color system.
An IC chip used in ordinary analog-to-digital conversion (A/D conversion) is of a type which effects the A/D conversion of data of three channels corresponding to three colors, i.e., R, G and B, and when an attempt is made to effect the A/D conversion of data of four channels by the use of this IC chip, two IC chips for three channels become necessary. Also, recently, many application specific integrated circuits (ASIC) containing an A/D-converting function therein have arrived on the market, and it is conceivable to effect the A/D conversion of the data of four channels by the use of these, but in such case, it is necessary to provide an IC chip for A/D conversion outside.